1. Field of the Invention
The present invention relates to the field of semiconductor fabrication, and more particularly, to chemical mechanical polishing and planarization of semiconductor devices.
2. Description of the Prior Art
The increasing need to form planar surfaces on a variety of materials has led to the development of process technology known as chemical mechanical polishing (CMP). The CMP method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A fixed-abrasive pad has abrasive particles held in a containment media, whereas a standard pad has a durable surface, without embedded abrasive particles. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Polishing slurry, including at least one chemically reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The polishing process is assisted by chemical compounds within the polishing slurry that facilitate removal of the material being polished. By carefully selecting the chemical components of the polishing slurry, the polishing process can be made more selective to one type of material than to another. The ability to control the selectivity of a CMP process has led to its increased use for delicate surface applications, such as the fabrication of complex integrated circuits.
A common requirement of all CMP processes is that the substrate be uniformly polished and that the amount of material removed by the polishing process be controlled in a repeatable fashion. Optical techniques have been developed to monitor the polishing process and to determine a process end-point. Typically, the optical end-point detection method involves generating a light beam and reflecting the light beam off of the surface being polished. Because both the surface being polished and the polishing pad are in continuous motion during the polishing process, it is difficult to construct an optical pathway for continuous light transmission.
Polishing pads are typically composed of two or more overlying layers of different materials. Typically, a polishing pad includes at least a polishing layer overlying a backing layer. Additionally, an adhesive layer is commonly used to adhere the backing layer to the polishing platen. Since the polishing layer and the backing layer are typically composed of different materials, the optical transparency of the materials also differs. Most materials used as a polishing layer are opaque to light over a wavelength range useful for end-point detection. Many of the materials used to construct a backing layer, however, are transparent to light. Accordingly, polishing pads have been fabricated in which sections of the polishing layer are removed and replaced with an optically transparent material to form an end-point detection window.
Copper damascene or copper dual damascene technique combined with copper/barrier CMP are known in the art. In a conventional copper dual damascene process, a dielectric layer is deposited onto a substrate, patterned, and etched back such that trenches, vias, and other recessed regions etched into the dielectric layer represent the desired metal interconnection pattern. Barriers and copper are then sequentially deposited/sputtered over the entire surface of the device, filling the recessed regions and blanketing the surface of the dielectric layer. The bulk copper layer and barrier layer are then polished back to a degree such that the Cu/barrier structure becomes electrically isolated within the recessed regions etched out of the dielectric material.
A typical copper/barrier CMP method includes the steps of (1) polishing the bulk copper layer using a first platen containing a hard polishing pad such as IC series pads (ex. IC 1000 or IC 1010) available from Rodel Inc.; and (2) removing the barrier layer using a second platen containing a soft polishing pad such as Politex. The Politex soft pad, which is prevalent in the semiconductor industry for the barrier CMP, is a poromeric pad having a hardness of about 60 (Shore A) and a compressibility of about 4.2% to 18%. It is noted that the Politex soft pad has no end-point detection window. Ordinarily, during the polishing of the barrier layer, no condition chemicals are dispersed on the Politex soft pad. The conditioning of the Politex soft pad is usually performed by means of a brushing tool, which is different from the way of conditioning for the hard IC series pads. The conditioning of the hard IC series pads is typically performed with a diamond disk in combination with condition chemicals dispersed on the pads.
However, the above-described prior art copper/barrier CMP utilizing a soft pad for barrier polishing has some drawbacks. First, the lifetime of the Politex soft pad applicable for the barrier CMP is very short (˜10 hours). Frequent change of the Politex soft pad on the second platen adversely affects the tool uptime and product throughput. Second, when using the Politex soft pad on the third platen, the removal rate of the barrier layer is unstable and the uniformity is not satisfactory. The unstable removal rate and poor uniformity due to the use of the Politex soft pad deteriorate as the pad is near the end of its lifetime.
In light of the foregoing, there is a constant need in this industry to provide an improved copper/barrier CMP process with sufficient reliability, increased throughput and relatively low cost.